The interest attracted by optical digital switching and computing is mainly stimulated by its potential to implement massively parallel architectures. This holds especially true for free space systems where three-dimensional space is the communication medium and where imaging setups use bulk optical elements such as lenses, mirrors, and beam splitters to interconnect two-dimensional arrays of optical logic devices. Exemplary systems of this type are disclosed, for example, in U.S. patent application Ser. No. 219,623 of J. Jahns et al., filed July 15, 1988, "Optical Crossover Network", and in U.S. patent application Ser. No. 248,468 of D. A. B. Miller et al., filed Sept. 23, 1988, "An Arrangement for Imaging Multiple Arrays of Light Beams", both applications being assigned to the same assignee as the present invention. These and other free space systems have one thing in common--the use of planar arrays of optical devices and corresponding arrays of light beams. Typically also, a number of different beam arrays are required because a usable logic device will, in general, need at least two logical inputs. Depending on the type of device, it may also require one or more optical bias beams. This is akin to a transistor logic gate, where one employs a number of logic signals and a power supply source for operating the logic gate.
In general, the optical devices employed in free space architectures require four or more ports: two signal or data inputs to set the state of each device, a power or clock input to read the state of the device, and a signal or data output comprising a reflected or transmitted power input modulated by the device. If the devices are connected in a switching network, for example, one or more additional ports are required for inputs used to control the switching nodes of the network. To avoid any interference or coherent artifact "noise", the inputs incident on each device may have orthogonal modes with respect to one another. Thus the inputs may differ in polarization, wavelength, time, or spatial location. Direction may also be an orthogonal mode; however, there is only one direction available for reflection devices (normal to the front of the device), and there are only two available directions for transmission devices (normal to the front and normal to the back of the device). The available orthogonal modes depend mainly upon the characteristics of the optical devices, e.g., polarization sensitivity, dual wavelength operation, etc., but the available optical hardware may also be a constraint. In addition, power and switching speed requirements dictate very small device sizes, requiring the use of diffraction-limited or near diffraction-limited optical imaging systems to interconnect the devices. For optimal performance from an an optical system of a given aperture, each input beam should use most of that aperture. Thus, lossless beam combination techniques that divide the aperture, i.e., pupil division, should not be used. These beam combination constraints limit the number of signal beams which may be fanned into a device without loss of power or resolution (space-bandwidth product). The number of available orthogonal modes also limits the number and type of interconnection operations that can be performed between two arrays of devices, since the use of a mode for signal interconnection directly affects how the signal beams will be combined with the power beam and separated from the output beam. Hence, there is a need in the field of free space optical systems to operate with a plurality of beams and, in particular, there is a need to arrange for multiple arrays of beams, with each array being derived from a different source or sources, to be incident on a desired array of optical devices. In other words, there is a need to combine beams and to separate beams.
The combination of a plurality of beam arrays may be effected in several steps--for example, first combining two beam arrays of different wavelength using wavelength-dependent or dichroic elements, and then combining the resulting beam arrays with other beam arrays using spatial position and polarization. It may be a requirement that the beams emerging from the wavelength-dependent combination have the same linear polarization type, e.g., p-type or s-type, so that they will be transmitted together through a polarization beam splitter used to effect further beam combination. In principle, an ideal dichroic beam splitter may be used to combine two beam arrays of different wavelengths without changing polarization. Thus if both beam arrays enter the dichroic beam splitter with p-type linear polarization, they will be both emerge with that same polarization. However, the performance of available dichroic beam splitters is not independent of polarization except over a narrow range of wavelengths. In optical setups where lenses use infinite conjugate imaging to relay images, the beams of an array may have a substantial angular field, e.g., plus or minus five degrees. Since the angular field corresponds to a wavelength range of the dichroic beam splitter, optical setups where the beams of an array have a large angular field may not be able to use a dichroic beam splitter to effect wavelength-dependent beam combination.
In view of the foregoing, a recognized need exists in the art for an apparatus to perform wavelength-dependent combination of beam arrays that have a substantial angular field, where the combined beams emerge having the same linear polarization type.